Patent Application
20060057502
1. Field of the Invention
This invention relates to a method of forming arbitrary patterned conductive circuits on boards with a metal colloid solution prepared by diffusing nano-sized metal fine particles into a solvent.
This application claims the priority of Japanese Patent Application No. 2004-215478 filed on Jul. 23, 2004, which is incorporated herein by reference.
There are a screen printing method and a resist-lithography method for producing wiring patterns on epoxy boards or other material boards. The screen printing method forms wiring circuits by preparing a screen having slits at positions corresponding to the positions where wirings should be made, fitting the screen onto a board, painting the screen/board with a conductive metal paste, heating and hardening the metal paste into permanent wiring patterns. What determines the patterns of wirings are the slit patterns inscribed on the screens. What eliminates the solvent out of the metal paste is the heating process. The heating treatment evaporates the solvent, hardens the resin including in the paste and fixes the metal patterns on the board. The printing method transcribes wiring patterns onto screen slit patterns. The metal paste has electric conductivity inherently. The metal paste does not acquires electric conductivity by additional crystal growth induced by heating in the screen printing method.
The lithography method forms wiring patterns on a board by covering the overall board with a copper thin film by evaporating or sputtering, painting a resist, exposing the resist through a mask with desired patterns by a mercury lamp, removing (ashing) the resist, and eliminating extra copper by etching.
Ultrafine patterns are also formed on silicon wafers by a similar lithography method containing the steps of making an aluminum thin film on a silicon wafer, spin-coating the Al-film with a rest, setting a mask with object patterns on the wafer, exposing the resist via the mask by ultraviolet rays, etching extra aluminum away and ashing the resist. Lithography provides semiconductor wafers or insulator wafers with wiring patterns by a set of the steps including painting of a resist, exposure via a patterned mask, removal of a metal film and ashing of the resist. What determines the wiring patterns are the patterns on the mask. The resist is indispensable for transcribing the mask patterns onto the metal thin film.
2. Description of Related Art
Wiring patterns on printing boards have been made by a screen printing method for a long time. Wiring patterns on silicon wafers have been made by a lithography method. Use of a screen (stencil) having slits inhibits the screen printing method for making very fine wiring patterns. The lithography method can depict fine wiring patterns having small line widths. Lithography, however, requires a large-scaled, expensive apparatus. The lithography method includes expensive steps, such as resist-painting, exposure, etching, and ashing.
Japanese Patent Laying Open No. 2001-35255, “Silver particle dispersion liquid” (Document 1) proposed a metallic paste dispersing silver fine particles or silver oxide fine particles in organic solvents which are mineral spirits, tridecane, dodecylbenzene, α-terpineol and so on. Document 1 insisted that the authors produced a mineral spirit dispersion liquid which contains 20 wt % of silver fine particles of, e.g., an 8 nm average diameter in mineral spirit, which shows a viscosity of 5 cP (centipoise) at room temperature. The authors of Document 1 produced another mineral spirit dispersion liquid which includes 20 wt % of palladium particles of a 6 nm average diameter in mineral spirit.
Document 1 produced a pattern on a Si wafer by spin-coating the Si wafer with the silver/palladium mineral spirit dispersion liquid, heating the coated Si wafer at 250° C. for two minutes in the atmosphere for evaporating an organic solvent, sintering the silver/palladium spirit at 300° C. for ten minutes, forming a 1 μm silver/palladium film, painting a resist on the silver/palladium film, exposing the resist via a photomask having a pattern, and eliminating unnecessary parts by development. Two steps of processing harden the silver/palladium paste. The first step is heating at 250° C. for two minutes for evaporating the organic solvent. The second step is heating at 300° C. for ten minutes for sintering the paste. The final product is similar to the wirings produced by aluminum evaporation. The paste film is converted to a conductived metal film. The conventional photolithography can depict patterns on the metal film.
At an early stage of semiconductor technology, wirings of LSIs (large-scaled integrated circuits) had been aluminum patterns produced by aluminum-sputtering and photolithography. Progress of miniaturization of wiring patterns reduced line widths which heightened electric current density. Extreme high current density had a tendency to induce occurrence of electromigration. Trials have been done for building copper wirings instead of the prevailing aluminum wirings. It is rare for copper to produce electromigration. But it is difficult to etch copper. Both wet etching and dry etching are inapplicable to copper. Incapability of etching excludes photolithography. Copper allows a new double-damascening method to make wirings by boring holes on an object substrate in advance, filling the holes with copper melt, solidifying the copper melt into copper solid in the holes, and erasing unnecessary copper projected out of the holes. The double-damascening method is a sophisticated, complicated and difficult method. Another candidate which has high resistance against electromigration is silver wirings. Trials of silver wirings have been done. Since silver is endowed with low resistivity, silver has been deemed to be an appropriate material for fine wirings. Uniform thick painting films are prepared by dispersing silver fine particles into an organic solvent in a uniform distribution silver solution, painting an object board with the silver solution, heating and drying the solution and making a uniform-thick silver films on the board. Then the use of photolithography can produce silver wiring patterns on the board. The silver solution is a novel material. But the following steps can be processed by the conventional photolithography which has ripened in the silicon semiconductor technology.
Japanese Patent Laying Open No. 2002-324966, “Method of forming circuit patterns by ink-jet printing” proposed a method of dispersing silver fine particles of 1 nm to 100 nm diameters into alkyl amines, adding thermosetting resin, e.g., phenol resin, producing a conductive metallic paste including silver particles, alkyl amines and the thermosetting resin, ejecting the conductive paste as fine drops from a scanning head of an ink-jet printer on an object board, depicting patters with a jet beam, and making paste patterns made of silver on the board. The line width is about 100 μm (0.1 mm). The thickness is about 5 μm. The paste patterns depicted by the ink-jet printer are annealed at 150° C. for 30 minutes for removing a dispersion agent and further annealed at 210° C. for 60 minutes for setting the thermohardening resin. The double steps of annealing are necessary to evaporate the dispersion agent and to harden the thermosetting resin (e.g., phenol resin).
The paste jetting method can directly depict wiring patterns by a scanning ink-jet printer with a metal paste instead of a printer ink. Movements of the printer head determine the patterns of wirings, which is similar to conventional printers. The paste jetting is a printing method which dispenses with screens (stencils). Resist painting, via-mask exposure and etching are unnecessary. The document (Japanese Patent Laying Open No. 2002-324966) asserted that the conductivity of the wiring patterns made by the paste jetting was 2.8×10−5 Ωcm.
The paste jetting method, however, has a drawback that two steps of long-term annealing are indispensable for eliminating the dispersion agent and hardening the thermosetting resin. A dot of the paste jet has a diameter of 16 μm to 20 μm. The dot size inhibits the paste jet method from depicting fine patterns having narrow line widths. The method cannot be applied to produce wirings having line widths narrower than 100 μm (0.1 mm).
Japanese Patent Laying Open No. 2003-156839, “Materials for mother patterns and a printing method by the mother pattern” does not relate to wirings but relates to a technique of making mother patterns which are negative origin patterns of transcription. A mother pattern is made by painting a metallic colloid on a sustainer board, irradiating selected parts of the metallic colloid with a scanning laser beam for revealing metal parts, and making desired metal-revealed parts. Unirradiated parts are left to be metallic colloid. Unirradiated metallic colloid is still hydrophilic. Irradiated metal-exposed parts are oilphilic. A printing ink adheres selectively to oilphilic parts. When the printing ink is supplied to the mother pattern, laser-irradiated oilphilic parts maintain the ink but unirradiated hydrophilic parts recoil the ink. The laser-irradiated oilphilic parts of the mother pattern are transcribed onto sheets of paper. The prior document (Japanese Patent Laying Open No. 2003-156839), which does not relate to the production of wirings, is here cited, because the method makes use of laser irradiation.
Conventional screen printing methods have a drawback of preparing slit-carrying stencils for painting metal pastes therethrough. Another drawback is the restriction of line widths. It is impossible to make narrow-width wiring patterns, because the widths are restricted by the widths of slits.
The lithography method comprising the steps of resist-painting, photolithography, etching and ashing is a sophisticated technology ripened in silicon semiconductor industries. The lithography method requires photomasks, resists and a large-scaled exposure apparatus for exposing masked resists.
The exposure apparatus demands a large sum of equipment investment from device makers. Use of resists requires an ashing apparatus for eliminating the resists after etching. Unselected metal parts still adhere to boards. Water-washing cannot remove the unselected metal parts from the board. Dry-etching or wet-etching is indispensable for removing unselected metal parts. The etching apparatus is also large-scaled and expensive, which raises the cost. Etching gases or etching agents, which are poisonous and harmful, have a tendency to induce environmental pollution.
A method which is immune from the abovedescribed defects is required to form various fine wiring patterns on a board with a high degree of freedom at low cost. This invention aims at proposing such a low cost wiring patterning method.
The present invention is a wiring method of preparing metal dispersion colloid by suspending metal fine particles by a dispersion agent in a solvent, painting an object board with the metal dispersion colloid, drying the metal dispersion colloid into a thin metal suspension film by removing the solvent, irradiating selected parts of the thin metal suspension film with scanning or diffracted laser beams, growing metal fine particles into large metal grains on the laser-irradiated parts, enhancing the cohesion of the large grown metal grains to the board and inducing conductivity to the large grown metal grains, washing unselected fine particles away from the board and forming conductive wirings of desired patterns on the board.
The wirings formed on the board by the present invention include metal particles with diameters of 30 nm (0.03 μm) to 10000 nm (10 μm). Porosity is 0.01% to 10%.
Here, the metal, which forms conductive wirings, should have high resistance against oxidization and high electric conductivity. Noble metals are suitable. The best candidate is silver. The next best candidate is gold. Other noble metals are also available. Silver (Ag), gold (Au), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), copper (Cu), nickel (Ni) and their alloys are appropriate metals for forming metal dispersion colloid. Alkali metals (Na, K, Li, Rb, Cs, etc.) or alkaline-earth metals (Ba, Ca, Sr, etc.) are less suitable. It is difficult to keep fine particles of alkali or alkaline-earth metals suspended in any solution since they have a strong tendency of oxidization. Aluminum (Al) is inappropriate. Fine aluminum particles are dangerous owing to rapid oxidization. Gallium (Ga) and indium (In) are also cunsuitable by the same reason as aluminum. Iron (Fe) is improper to a metal suspension due to fast oxidization and low electric conductivity.
Here, the fine particle means a particle with a diameter from 0.5 nm to 200 nm. Since the diameters are subnanometer or nanometers, the fine particles are sometimes called “nanoparticles” in the present description. Nanoparticles are endowed with high activities owing to large ratios (S/V) of surface areas (S) to volumes (V). A preferable range of diameters of nanoparticles is from 1 nm to 30 nm.
The word “solvent” is here used for a special meaning slightly different from the conventional usage. Conventional solvent solves an object material (solute) and fully extinguishes solid parts of the object solute. The solvent, however, means a fluid liquid which allows fine solid particles to be suspended with inherent sizes and shapes therein. The solvent neither reacts with nor ionizes the nanoparticles. No chemical reaction takes place between the solvent and the nanoparticles. Appropriate solvents are water (H2O) or alcohols. The solvent in the present description denotes a liquid which includes metal nanoparticles and has fluidity sufficient to paint surfaces of object boards.
If an alcohol were employed as a solvent at early stages, reduction would be induced by a high reduction tendency of alcohols. Thus the first proper solvent should be water (pure-water). After the colloid has been reduced, large metal particles have appeared and obstacle anions have been removed, alcohols can be added to the solution as an additional solvent.
The dispersion agent is an organic material for preventing metal fine particles from aggregating in a liquid and maintaining metal fine particles floating in the liquid. Metal fine particles have a high surface activity and have a tendency to aggregate for reducing the surface activity. If metal fine particles were supplied into water or alcohol, particles would aggregate together into large grains and would lose floatage and fluidity. Spin-coating of the particles/water or particles/alcohol cannot form uniform thin films on boards due to irregular aggregation. The dispersion agent which prevents metal fine particles from cohering is indispensable for making metal colloids.
The chemicals, which work as a dispersion agent, are polycarboxylic acid type macromolecular anions or chemicals having a molecular weight more than 150 and having strong adhesion to metal fine particles. For example, the dispersion agents are dodecyl amine (Molecular weight: Mw=185), stearyl amine (Mw=270), oleil amine (Mw=267), stearic acid amide (Mw=287), Oleic acid amide (Mw=281), myristic acid (Mw=228), lauric acid (Mw=200), palmitic acid (Mw=256) and so on. Dispersion agent molecules encapsulate each one of metal fine particles. Dispersion agents prohibit fine particles from cohering together. Fine particles are isolated from each other in a solvent by the enclosing dispersion agents. The metal fine particles maintain the separated state. Cohesion of metal particles is hindered by the dispersion agent. Preferable dispersion agents are chemicals which excel in thermal-decomposition property. Appropriate dispersion agents should have a critical temperature lower than 450° C. at which the weight goes down to 80% in heat weight analysis at a heating rate of 10° C./min under the atmosphere. It is much better that dispersion agents have a critical temperature less than 400° C. for inducing a weight loss of 20% in the same heat wight analysis.
Polycarboxylic acid type macromolecular anions sold on the market are, e.g., EFKA5071 (produced by EFKA Chemical Corporation), Flowlen G-700 (produced by Kyoeishakagaku Corporation), Flowlen TG-750W (produced by Kyoeishakagaku Corporation), Flowlen G-700DMEA (produced by Kyoeishakagaku Corporation), Softanol (produced by Nipponshokubai Corporation), Selna D735 (produced by Chukyoyushi Corporation) and Discoat N-14 (Daiichikogyoseiyaku Corporation).
The metal dispersion colloids are suspension liquids having water (or alcohol) and metal fine particles suspended in water. The metal dispersion colloids can be stored for a long term in glass bottles or plastics bottles without precipitation and degradation.
Object boards are painted with metal dispersion colloids. The object boards are glass boards, ceramic boards, epoxy boards, polyimide boards, polyethylene terephthalate (PET) boards, silicon (Si) wafers, gallium arsenide (GaAs) wafers, indium phosphide (InP) wafers, silicon dioxide wafers (SiO2) and so on.
If a step of heating objects up to a high temperature were included in a method, the method could not be applied to plastics or low-melting point glass. This invention succeeds in keeping the object boards unheated by converging the laser beam only on the metal colloid. This invention is applicable to plastic boards and low-melting point glass boards. The scope of applicable object boards is enlarged due to the exclusion of heating steps. This is one of the features of the present invention.
This invention includes neither wet etching nor dry etching. Rigidity, refractoriness, sturdiness and resistance against chemicals are not required of object boards. The metal dispersion colloid is dried. Object boards should pass in the drying process. The drying temperature is about 100° C. The drying time is ten minutes to twenty minutes. Most plastics boards are acceptable. If a candidate board degenerates even at 100° C., the present invention can be applied by lowering the drying temperature down to 80° C. The present invention is available for almost all boards of plastics, woods, glass, metals, ceramics and crystals. This is a great advantage of the present invention.
The metal dispersion colloid can be painted on boards by a brush, a spinner and so forth. Painting techniques of the metal dispersion colloid on object boards are spin-coating, doctor blade coating, rolling coating, spray coating, dipping coating, screen printing and ink-jet printing and so on.
The boards painted with the dispersion colloid are dried for eliminating the solvent and depriving the colloid of fluidity. Room temperature drying or hot window-blowing drying is available. Drying eliminates only the solvent (water or alcohol) without chemical reaction. Dried metal suspension films have no electric conductivity. Metal nanoparticles are separated by dispersion agent molecules from other nanoparticles. The dispersion agent is an inherent insulator. Thus no current flows in the dried metal suspension film. The metal suspension film has poor adhesion to the board. If the film were washed by water, the metal suspension film would easily be eliminated from the surface of the board.
Then the metal suspension film is partially irradiated with a strong laser beam or beams. The power of the laser beam evaporates or solves dispersing agent molecules. Since the separation of the dispersion agent molecules is removed, metal fine particles come in direct contact with each other. Localized sintering is induced by the laser power. The sintering facilitates the fine particles to aggregate and grow to be larger metal grains. The Inventors confirmed by electron microscope observation that the fine particles cohere to bigger granules on the parts irradiated by the laser beam. Growth into large metal grains rapidly reduce electric resistance. Electric conductivity is given to the suspension film of the laser-irradiated parts.
Enlarged grains by the growth are endowed with tight cohesion to the board on the irradiated parts. The irradiated parts of the film permanently adhere to the board. Water cannot wash the irradiated parts away. The irradiation of laser beams gives both board-coherence and electric conduction to the metal suspension film. However, the bestowal of coherence and conduction requires large power of laser beams. As explained later, Embodiments employ a laser beam of 450 mW power. The 450 mW laser beam can be obtained by gathering power of hundreds of current InGaN lasers.
Laser light evaporates dispersion agent molecules. Vacancies are made among metal particles. The laser beam induces metal particles to aggregate for filling vacancies and to grow to be larger grains. The laser beam bestows growing grains the coherence to the underlying board. Both the dispersion agent molecules and the metal fine particles absorb the laser light. Big metal grains do not absorb but reflect away visible light. Fine particles of metal induce random- and multi-reflection of laser light. Many times of reflection enable the fine particles to absorb laser power with high efficiency. The metal dispersion colloids of the present invention look black. High absorption rate gives darkness to the metal dispersion colloids.
It is supposed that photons included in laser beams would cut the chemical bonds between metal fine particles and dispersion agent molecules. The dispersion agent molecules are freed from the metal fine particles. Laser power heats, evaporates, solves and removes the dispersion agent molecules. Extinction of the dispersion agent molecules would be caused by evaporation or dissolution. Loose chemical bonds between the dispersion agent molecules and the metal nanoparticles should be cut before the extinction. Decoupling of the chemical bonds requires high power density of the laser beams. Even if a high power density laser were obtained, laser irradiation being not absorbed by the metal dispersion colloids would be unoperative yet. Thus absorption spectra of the metal dispersion colloid and the dispersion agent are measured.
Which should be irradiated, dispersion agent molecules or metal fine particles? This is a problem. The Inventors of the present invention thought that it would be effective to irradiate metal fine particles with light for promoting the cohesion of metal fine particles and the crystal growth of metal. The Inventors had an idea for making use of a laser which emits a wavelength ranging between 350 nm and 490 nm as a light source. 350 nm to 490 nm wavelengths correspond to ultraviolet/violet/blue rays. The dispersion agent cannot absorb ultraviolet/violet/blue rays. Instead, metal particles absorb the light of the wavelength range (350 nm-490 nm), and raise the temperature. An effective wavelength range of lasers turns out to be 300 nm to 550 nm in general in the present invention. Enhancement of the temperature evaporates or decomposes the dispersion agent molecules. Extinction of the dispersion agent allows metal particles to cohere into large grains and to stick to the base board. Once the metal grains have glued to the board, washing cannot erase the metal grains away from the board.
The Inventors hit on a good idea of making the best use of InxGa1-xN (abbr. InGaN) type LEDs or LDs as a light source for emitting light of a wavelength between 350 nm and 490 nm. The Inventors chose 408 nm wavelength InGaN semiconductor lasers as a light source. InGaN lasers can make light of an arbitrary wavelength from 350 nm to 490 nm by varying the mixture crystal ratio x in InxGa1-xN. Embodiments described later will adopt 408 nm wavelength laser diodes. The total power is 450 mW in Embodiments.
No single laser diode can make such big power of 450 mW. The Inventors produced a large power source by preparing N InGaN laser diodes (N is a large number) and an N:1 fiber coupler, coupling N laser diodes (LDs) with N ends of the fiber coupler, introducing LD beams into the N:1 fiber coupler, and gathering all beams emanating from the LDs into a single fiber. For example, an assembly of a hundred of high power laser diodes of 4.5 mW produces a strong light source of 450 mW. Another set of two hundreds of laser diodes of 2.25 mW will give another large power light source of 450 mW.
In practice, the LD assembly incurs loss of power at junctions. The real sum of power of the LD assembly is smaller than the nominal sum. An arbitrary number of LDs can be joined by optical fibers in the same direction. A strong light source having arbitrary desired power can be built by unifying a large number of identical laser diodes by the fiber coupler. Tiny laser diodes are congenial to slender optical fibers. A hundred laser diodes, two hundred laser diodes or so can easily be unified by optical fiber couplers.
The present invention forms a desired wiring pattern on a board by painting a surface of the board with a metal dispersion colloid including metal nanoparticles, drying the metal dispersion colloid into a thin metal suspension film, irradiating the metal suspension film by a laser beam capable of depicting a desired pattern, hardening the irradiated parts of the suspension film, washing unirradiated parts away, and maintaining a wiring pattern of the laser-irradiated parts on the board.
The metal suspension film which has been painted and dried lies ephemerally on the board. Only laser-irradiated parts cohere to the board. Unirradiated, unnecessary parts are easily eliminated by washing, for example, supersonic washing. This invention dispenses with etching. Scanning of a laser beam enables the present invention to depict an arbitrary wiring pattern on a board. The laser beam scanning which excels in high degree of freedom of design, is suitable for making unroutine wiring patterns. Otherwise, diffractive optical elements (DOEs) allow the present invention to form a desired complicated wiring pattern without scanning of laser beams.
Irradiation of laser beams brings about conductivity and coherency to the suspension film. Annealing following the irradiation is entirely unnecessary. Elimination of post-annealing is one of the features of the present invention. Shortening of a focal depth of a beam guiding optical system enables thin suspension films to absorb all the laser power, and inhibits laser power from arriving at boards. The boards are left unheated in the case. This invention is applicable to plastic boards or low-melting glass boards which have poor resistance against heat. A wide scope of applicable boards is another feature of the present invention.
A prior technique of making wiring patterns is a screen printing method. The present invention is superior in fineness of wiring patterns to the screen printing method which paints a screen (stencil)-covering board with a metallic paste with a painting roller. The minimum width wiring patterns is equal to the diameter of a laser beam. Converging a laser beam narrower allows the present invention to form a wiring pattern having narrower widths. The present invention can eliminate the cost of fabricating the screens (stencils) which have many slits patterned after an object wiring.
Another prior technique of depicting wiring patterns is a lithography method including resist-painting, photolithography, etching and ashing, which is prevalent in semiconductor wirings. In comparison with the lithography method, the ink/irradiation method of the present invention has advantages of elimination of photomasks, resists, exposure of resists, etching and ashing of resists. The ink/irradiation method can do without a large, expensive exposure apparatus which produces strong-power rays of short wavelengths for exposing resists. The lithography method requires etching processes for removing unnecessary parts. The ink/irradiation method (this invention) can remove unnecessary parts only by washing with water. The present invention dispenses with poisonous etching gases, etching liquids and an expensive etching apparatus. The present invention can eliminate the ashing process of removing resists. What depicts wiring patterns is a laser optical system. The steps of making wirings on boards are simplified by the ink/irradiation method. Current design rules on silicon wafers are 130 nm or 65 nm. The present invention does not aim at such an ultrafine wiring pattern. Object patterns of the present invention have line widths ranging from micrometers to hundreds of micrometers.
Object wirings of the ink/irradiation method have widths far broader than the current design rule. The minimum of the wiring widths is determined by the diameter of a laser beam on the board. In the case of guiding a laser beam by an optical fiber, a fiber core size and a reduction rate of an optical system determine the size of the laser beam on the board. A multimode fiber with a wide core inhibits the laser beam from writing narrow width wirings even at a small reduction rate.
Adoption of a single-mode fiber of a 10 μm diameter core and a reduction rate of 1/20 enables the laser beam to depict fine patterns having a 0.5 μm width. When the width of object wirings is so narrow, the property of the metal wirings depends upon the size of metal fine particles. Ultrafine particles having diameters of 0.5 nm to 10 nm should be chosen. But writing submicron wirings is not a main purpose of the present invention. The main purpose is to make wirings having widths wider than 1 μm and narrower than 1 mm.
Fourteen sorts of metal dispersion colloid including metal nanoparticles are described. Materials are listed first in each explanation. Water is a solvent. Metal is supplied to water by a metallic acid including metallic ions. Mixing of the materials makes a solution containing metallic ions as cations in the acids. Metallic cations are fully dissolved in water. A cation is not electrically-neutral but positively-charged. No powder exists in the solution. In advance, a dispersion agent is added to the solution for preventing metal powder from cohering together. Then a reducing agent is supplied to the solution. The solution is stirred. The reducing agent is an agent for reducing metallic cations into metal powder which is electrically neutral. Reduction means giving a unit negative charge. Oxidization, which is an antonym of reduction, means giving a unit positive charge. Metallic cations are reduced to metallic fine powder in the solution. Metallic powder contains nanoparticles of metal. Appearance of metal neutral nanoparticles gives a color inherent to the metal to the solution. After the reduction of metallic cations into nanoparticles, anions of the acid still remain in the solution. The anions are obstacles. The anions should be eliminated by electrodialysis or centrifugal separation. At the final stage, surface tension and viscosity are adjusted by adding water, organic solvents and coupling agents. The final product is a liquid including metal nanoparticles, dispersion agent molecules and a solvent (water in many cases). Each of the metal nanoparticles is encapsulated by dispersion agent molecules. Nanoparticles have high inherent surface activity for cohering together. Dispersion agent molecules separate nanoparticles by encapsulating. Enclosure by the dispersion molecules hinders nanoparticles from cohering. Nanoparticles accompanied by the dispersion agent molecules are floating in the solvent. Such a liquid is called a colloid or a metal dispersion colloid in the description.
As clearly explained above, metal dispersion colloid is prepared by adding a reducing agent to an acid solution containing metallic ions and reducing metallic ions into metal fine grains suspended in a colloid. An important matter is an early supply of a dispersion agent at a starting step into the acid solution. If a dispersion agent were supplied into the acid solution after adding a reducing agent, aggregation would start and would prevent metallic ions from forming ultrafine metal nanoparticles. Anions composing the acid remain in the solution containing the nanoparticles. The anions are unnecessary. The anions are eliminated by electrodialysis, centrifugal separation or margin filtering. If once of electrodialysis etc. cannot eliminate all the anions, once more, twice more times of electrodialysis should be repeated till the whole of the anions have been removed.
The light wavelengths which enable metal dispersion colloid to harden and metal nanoparticles to aggregate together should be 300 nm to 550 nm, which has already been described. More favorable wavelengths are 350 nm to 490 nm.
InGaN semiconductor lasers can produce light having a wavelength in the above range. But an InGaN laser is too weak to induce the chemical reaction in the metal colloids. A light source of large power can be obtained by coupling many InGaN lasers by optical fibers to be a single source. A set of several tens or hundreds of InGaN lasers can be a strong light source enough to induce the desired reaction in metal suspensions. The wavelength of InGaN lasers can be varied in the preferable range between 350 nm and 490 nm by changing the mixture ratio of In and Ga. The above Embodiments adopt 408 nm InGaN lasers (λ=408 nm).
There are many laser irradiating optical systems suitable for exposing the metal suspension films.
[System (1) (
A laser beam is introduced into an optical fiber. The optical system converges a diverging laser beam emitted from the optical fiber into a light spot on the suspension-coated board. Power distribution of the laser beam is transferred to the power distribution of the light spot by the optical system. As shown in
[System (2) (
A laser beam is expanded into a wide parallel beam by collimator lenses. The wide parallel beam is converged into a small light spot on the suspension-carrying board by a converging lens.
[System (3) (
System (3) is built by adding a diffraction type or refraction type homogenizer and a lens between the lens and the board of System (1) of
[System (3′) (
System (3′) is built by adding a diffraction type or refraction type homogenizer befored the lens to System (2) of
In
[System (4) (
System (4) is built by endowing a homogenizer DOE with a converging function and eliminating the rear lens from System (3) of
[System (4′) (
System (4′) is built by providing a homogenizer with a converging (Fresnel) function and eliminating the lens from System (3′) of
[System (5) (
System (5) contains a refractive or diffractive beamshaper which can convert an input beam into an arbitrary pattern-distribution beam on the suspension-carrying board. The beamshaper can be composed of aspherical lenses or a diffractive optical element (DOE). In
[System (5′) (
System (5′) contains a refractive or diffractive beamshaper converting an input beam into an arbitrary pattern-distribution beam. In
[System (6) (
System (6) contains a Fresnel-type refractive or diffractive beamshaper which can convert an input beam into a converging arbitrary pattern-distribution beam. The “Fresnel-type” means the possession of the converging function. The Fresnel beamshaper can be composed of aspherical lenses or a diffractive optical element (DOE). In
[System (6′) (
System (6′) contains a Fresnel-type refractive or diffractive beamshaper converting an input beam into a reducing arbitrary pattern-distribution beam. In
[System (7) (
System (7) contains a set of galvanomirrors which reciprocally sway in x- and y-directions for scanning an input beam in two-dimensional space. The galvanomirrors can produce scanning continual linear patterns in x- and y-directions on the board from a continual laser beam. The galvanomirrors can produce series of intermittent dots in x- and y-directions on the board from a pulse laser beam. Use of an fθ lens enables constant angular velocity galvanomirrors to depict equi-spaced distribution of dots.
[System (7′) (
System (7′) contains a set of galvanomirrors swaying in x- and y-directions. The galvanomirrors can produce scanning continual linear patterns in x- and y-directions on the board in the case of a continual laser. The galvanomirrors can produce series of intermittent dots in x- and y-directions on the board in the case of a pulse laser.
[System (8) (
System (8) contains a beamsplitting DOE which has a function of dividing a single beam into a plurality of beams. System (8) inserts the beamsplitting DOE in a wide parallel beam for making a plurality of beams and depicting plural dots on the suspension-carrying board. Use of the beamsplitting DOE is profitable in the case of irradiating many points with laser beams simultaneously. In
[System (8′) (
System (8′) contains a beamsplitting DOE which has a function of dividing a single beam into a plurality of beams. In
Polyimide boards, polyethylene terephthalate (PET) boards, glass boards, and ceramic boards are prepared as base boards. The specimen boards are spin-coated with the silver suspension ink as described in the metal suspension colloid 1.
(Spin-Coating)
A specimen board is vacuum-locked on a disc rotor of a spinner. Metal suspension colloid is dropped on the specimen. The rotor is rotated for thirty seconds at a speed of 200 rpm to 3000 rpm for making a thin suspension colloid film on the specimen board.
(Drying)
The specimen is dried at 100° C. for 10 minutes. Water (solvent) is fully removed from the metal suspension film. The disperse agent still remains in the metal suspension film. The suspension film has a thickness of about 0.3 μm. The electric resistance is high. The specimen films are insulators. The colloid film is unstable. Washing with water can fully eliminate the suspension film from the specimen board.
(Laser Irradiation)
The optical system shown in
The experiment is explained in detail. Object boards are polyimide boards. 0.3 μm thick silver suspension films are made on the polyimide boards by painting the polyimide boards with silver dispersion colloid and drying the dispersion colloid into a suspension film. Linear parts of the suspension film are scanned several times by an irradiating beam of a laser of a 408 nm wavelength and 450 mW power for evaporating the dispersion agent. The scanned part has a 13 mm length and a 0.18 mm width. Silver nanoparticles are aggregated and reinforced in the scanned linear parts. The suspension film is washed for eliminating unexposed silver nanoparticles. Irradiated silver suspension lines remains on the board. Resistances are measured between two ends of the silver suspension lines of different scanning times. Electric resistivities are calculated by multiplying the resistances by the thickness and the width and dividing by the length.
Surfaces of metal dispersion colloid on the boards are observed step by step by an electron microscope.
Electric resistances are measured between both ends of the 13 mm long sliver-suspension stripe after one shot, three shots, five shots and seven shots of scanning laser beams. The measured resistances are reduced into electric resistivities by multiplying the resistance by the width and thickness and by dividing by the length of the silver stripe. The result of the measurements are listed in Table 1.
Table 1 shows a result of measurements. The leftest first column denotes the laser beam irradiating (scamiing) times. The second column shows lengths (mm) of laser beam scanning loci on the board. The third column denotes widths (μm) of the laser beam scanning loci. A 180 μm width of wiring stripes seems to be quite wide for wiring patterns. Such a far wide metal stripe is suitable for measuring resistivity of the metal pattern exactly. Reduction of the magnification rate of the optical system narrows wiring pattern widths.
Narrower patterns enable fewer times of laser scanning or weaker laser power to aggregate metal nanoparticles into larger polycrystal grains and to reduce resistivity.
The diameter of the scanning laser beam spot determines the minimum of the depictable wiring widths. The beam spot size depends upon the laser optical system for converging the laser beam into a light spot on the object board. The size of a laser beam just emitted from an optical fiber is equal to the diameter of the core of the optical fiber. Reduction ratio of the optical system determines the ratio of the spot size to a core size of an fiber. When the core of an optical fiber is 400 μm in diameter and the reduction ratio is 1/2, the minimum wiring width can be 200 μm. When the reduction rate is 1/10, the minimum wiring width can be 40 μm. When the core of an optical fiber is 10 μm and the reduction rate is 1/10, the minimum wiring width can be 1 μm.
The fourth column means thicknesses of metal wiring patterns made on the board. The fifth column is electric resistance (Ω) of the metal wiring patterns. An increment of laser scan times reduces resistances of metal film lines. The sixth column is electric resistivities (μΩcm) of the wiring patterns. Electric resistivity is calculated by dividing the measured resistance by the length and multiplying by the section (thickness×width) of the pattern.
| Number | Date | Country | Kind |
|---|---|---|---|
| 215478/2004 | Jul 2004 | JP | national |
